Electronic imaging and projection systems typically include several color channels, each of which provides light of a different spectral band (e.g. red, green, and blue). Light from each spectral band constitutes a color component of the intended image. The component color images are mixed (combined) to form a composite multicolor image. Different techniques may be used to mix the component color images. For large-scale systems, separate modulation and projection optics are typically used for each component color image and the component color images are mixed through convergence of the component color images onto a screen or other display surface by focusing optics. For compact and portable systems, however, it is often preferred to combine component color images through coaxial mixing. With coaxial mixing, the component color images originating from each color channel are combined and merged onto a common optical axis before delivery to a screen or display surface. Coaxial mixing of component color images conserves space and cost by using one set of projection optics for the different color channels and through sharing of modulation and optical components by the different color channels.
Early electronic imaging systems employed lamps and other polychromatic light sources to provide the colored light of each color channel. Several different light-mixing systems were developed to combine component color images in the early systems (e.g. complex prism arrangements adapted from color television camera optics). With the advent of more powerful and more nearly monochromatic light sources (e.g. light emitting diodes (LEDs) and lasers), it became possible to reduce the size and cost of color mixing components, as well as to improve color gamut, optical efficiency, and overall performance of the imaging system. For color mixing, various arrangements of composite prisms and dichroic coatings were developed for use in projectors and similar imaging systems. Among the more familiar solutions used with earlier electronic imaging systems are X-cubes or X-prisms (shown in FIG. 1A), and related dichroic optical elements, and Philips prisms (shown in FIG. 1B). Non-prism solutions include sets of angled dichroic plates, such as those as proposed in U.S. Pat. No. 6,676,260 (Cobb et al.) entitled “Projection Apparatus Using Spatial Light Modulator with Relay Lens and Dichroic Combiner.”
Referring to FIG. 1A, an X-cube 10 is a composite prism, formed from four separate prism elements 10a, 10b, 10c, and 10d, as shown in inset E1, that are coated and then glued together to form a single color combiner element. The X-cube combines light from three solid-state light sources 14r, 14g, and 14b, such as laser diodes, emitting red, green, and blue light respectively. As assembled, X-cube 10 has two inner crossed dichroic interfaces 12a and 12b that are treated to selectively reflect and transmit different wavelengths. Dichroic interface 12a reflects blue wavelengths and transmits green and red wavelengths. Dichroic interface 12b, contiguous to dichroic interface 12a so that the two interfaces intersect along a line through the center of the X-cube, reflects red wavelengths and transmits green and blue wavelengths. The line of intersection of the dichroic surfaces is orthogonal to the plane of the drawing as X-cube 10 is represented in FIG. 1A. The colors are combined onto an optical axis OA, shown with separate color paths for clarity in FIG. 1A, but coaxial in practice.
The Philips prism 70, shown in FIG. 1B, is a more complex composite prism that is used for combining colors. Philips prism 70 is formed from three separate prisms, prism elements 70a, 70b, and 70c, and includes an air gap 72. An arrangement of dichroic surfaces at oblique angles directs light from solid-state light sources 14r, 14g, and 14b onto optical axis OA.
Dichroic surfaces are formed from stacked layers of ultra-thin coatings of various dielectric materials and can be formulated to provide selective reflection and transmission for light of various wavelengths. In the X-cube and Philips prism devices, and other related spectral combiners or separators, various types of dichroic coatings provide the color-selective mechanism that allows light to be spectrally re-combined or separated in a highly controlled manner.
Small, hand-held projectors and various types of embedded or accessory projectors typically use an arrangement of dichroic surfaces for mixing red, green, and blue laser light onto a single optical axis. These devices then rapidly scan the resulting light onto a display surface. To minimize battery usage and heat generation, these devices form each pixel by directly modulating each of the lasers, thereby producing only the light that is used to form the image itself. These projectors form an image by generating successive scanned lines of pixels that are then directed to the display surface.
While hand-held projection devices achieve good results using conventional color combination techniques, a number of problems remain. One problem with conventional color combination using dichroic surfaces relates to incident angles. Dichroic coatings reflect and transmit light as a function of incident angle and wavelength. As the incident angle varies, the wavelength of light that is transmitted or reflected also changes. For light that is incident at low angles (angles close to normal), the variation in response over small ranges of incidence angle or wavelength can be very low or negligible. For light incident at larger angles, however, variation in response over a range of angles can be pronounced, compromising dichroic coating performance. These coatings work best at small incidence angles, relative to normal, and it can be expensive and difficult to design and fabricate a dichroic coating that gives uniform results with incident light at large angles or over a wide range of angles. Where a dichroic coating must accept light over a large range of angles, perceptible color non-uniformities can easily result.
Other drawbacks of existing color combiners relate to the number of surfaces upon which the light is incident, whether it reflects from or transmits through the surface. Each surface of incidence represents an efficiency loss and leads to a reduction in brightness. In addition, each reflection of light from or transmission of light through a dichroic surface causes some loss, due to imperfect dichroic performance. Contrast is also compromised when light encounters a surface (whether coated or not) due to leakage. In the X-cube 10 in FIG. 1A, each of the light beams is incident on at least three surfaces and in the Philips prism 70 in FIG. 1B, each of the light beams is incident on at least four surfaces. The high number of surfaces compromises the performance of the X-cube and Philips prism devices for color mixing.
Another consideration for dichroic coatings relates to reflection and transmission of polarized light. As incident angles for light on the dichroic surface become larger, the differential in reflection efficiency for different polarization states becomes increasingly pronounced. Moreover, when incident polarized light originates from multiple laser diode sources, the unique polarization characteristics of the different laser diodes must be taken into account. One or more phase retarders can be added to the different color channels, but only at the expense of added cost and complexity. The task of designing and fabricating dichroic coatings that allow combinations of laser light of different wavelengths but the same polarization presents a considerable challenge.
A further complication in the mixing of colors provided by laser sources relates to the dimensional characteristics of the laser beams. The cross-sectional shape of many solid state laser beams is anamorphic: the beam cross-sections are more elliptical in shape than circular and the degree of elliptical distortion typically varies from one color to the next. This makes it difficult to form uniformly sized pixels from three component colors and leads to a degradation in color quality.
Cost is another concern with conventional color mixers. As FIGS. 1A and 1B show, conventional devices for color mixing include complex arrangements of dichroic surfaces and prism elements. A number of fabrication and precision assembly operations are required for their implementation. In conventional manufacture, two or more separate prism elements are formed, dichroic coatings are applied to one or more outer surfaces of the prism elements and two or more prism elements are then glued together, or otherwise joined in a precise geometric arrangement. In a typical prism assembly, one or more dichroic coatings are in the interior of the color combiner, surrounded by glass or other transparent material that forms the different prism elements. Color mixing performance can be compromised by coatings tolerances, slight misalignments in prism positioning, air gaps, surface imperfections, and other factors.
There remains a need for a color mixing solution for solid-state light sources that minimizes cost, minimizes the number of surfaces upon which light from different color channels are incident, reduces angles of incidence, and improves polarization response.